1. Field of the Invention
The present invention relates to control of fixing an image formed by an electrophotographic method.
2. Description of the Related Art
In an image forming apparatus that thermally fixes, on transfer paper, a toner image formed by an electrophotographic method, in general, the fixing temperature of a fixing device is determined in accordance with the amount of applied toner per unit area on the transfer paper. For example, the maximum value of the amount of applied toner per unit area is determined in advance, and the temperature of the fixing device is adjusted so as to obtain a fixing temperature capable of reliably fixing an image having the maximum value. Recently, however, power consumption is required to be lower because of growing environmental awareness. When the amount of applied toner changes depending on the image of the page to be output, it is necessary to perform energy-saving fixing temperature control capable of adjustment to a temperature at which reliable fixing can be performed on a page basis.
The image forming apparatus may use an image composition function (to be referred to as image composition hereinafter) of composing an image with a form image stored in advance and outputting a composite image. The form image is generated in advance by, for example, scanning an image printed on paper and stored in a storage device such as an HDD in advance. When such image composition is performed for read data of an original, the amount of applied toner of the image after image composition is different from the amount of applied toner calculated from the read data. Hence, to perform more appropriate fixing temperature control, the image composition needs to be taken into consideration. Japanese Patent Laid-Open No. 6-110354 (patent literature 1) discloses a technique that prepares two fixing units of different set temperatures, and when using image composition, performs fixing using the fixing unit of the higher set temperature.
However, employing the arrangement including two fixing units to obtain an appropriate fixing temperature, as described in patent literature 1, leads to an increase in the size or cost of the image forming apparatus. For this reason, when performing image composition, it is preferable to detect the amount of applied toner of the image after image composition and control the temperature of the fixing unit. In general, however, the load of applied toner amount calculation processing is heavy, and performance is expected to be low when the processing is implemented by software processing.